1. Field of the Invention
The present invention relates to electrical contact referred to as working contacts, and to a process of manufacture thereof.
The working contact is used, for example, in a relay of an electric circuit and interrupts the flow of the electric current passing through the circuit at the breaking opeation. The electric discharge, which takes place at the making and breaking operations, brings about one important phenomenon of the working contact, i.e. the erosion of the contact. The erosion of the contact principally results from the transfer of the cotact material. The transfer of material means a movement of metallic material such that a metal of one contact member gradually moves to the opposite contact member across the contacting surfaces thereof, as the contact repeats the making and breaking operations. The transfer of material results from:
(1) a short arc generated immediately before connecting and after disconnecting the contacts. PA1 (2) melting of the material due to the Joule heat caused by the electric current passage across the contacting portions of the contact and depending upon the contact resistance of said portions, and; PA1 (3) a bridge formation when the contact initiates the breaking.
When the contact begins to be subjected to erosion, metal of one of the electrodes, i.e. contact members, transfers to the other electrode with the result that the transferred metal concentrates, i.e., collects on said other electrode. Pips and craters are, accordingly, formed on, for example, the cathode and anode contacts, respectively. As the pips and craters grow, the separation of the contact becomes degraded and it finally loses its function as a contact. In this state, the contact fails and its usefulness as a contact is terminated.
As stated above, erosion of the electrical contact is critical to the life thereof and, thus, it is important to avoid such erosion. It is generally recognized that erosion of the contact depends upon circuit conditions, such as the voltage, current density and load and, further, that erosion of the contact is lowered with decreased levels of the above circuit conditions. The kinds of materials used for the contact of a particular circuit have a large influence on the erosion of the contact. The materials to be used for the contact are, therefore, critical to the life of the contact members and, thus, various kinds of material for the electrical contact have been proposed.
Several noble metals having high melting temperatures are widely used as the electric contact members such as gold for the gold diffusion contact and the hard gold contact, rhodium, rhenium and rutenium. The known contacts, however, are not suited for working contacts operated at a medium level of circuit conditions.
There is provided in the British Patent Specification No. 1,048,520, a method for the manufacture of composite contact assembly consisting of a core and a sheath, wherein the core is made of either of copper, copper alloy, steel, nickel, or aluminum, and the sheath is constituted of brazing material selected from silver solders, tin and tin-lead solders. However, this composite contact is, also, not suited for the working contact described above.